Traditionally, superheterodyne receivers have utilized a frequency translation method to shift a frequency spectrum passed by a relatively wideband radio frequency (RF) front-end to a somewhat narrow passband of an intermediate frequency (IF) section. IF filters of the IF section of the receiver have generally provided selectively sufficient to isolate a desired signal and an IF amplifier has typically amplified the signal provided to a detector. Historically, FM reception systems have usually implemented a narrow IF filter passband to provide greater attenuation of adjacent channels. However, a narrow IF filter passband may corrupt both the amplitude and phase of modulation sidebands of the desired channel. An IF filter that corrupts the modulation sidebands of the desired channel typically generates distortion in a detected baseband audio signal.
Recently, FM reception systems have implemented dynamic IF filter bandwidth control. This feature allows IF filters to remain relatively wide when there is no adjacent channel interference. However, in the event that modulation sidebands of an adjacent channel begin to encroach into a desired channel, the IF filter narrows to mitigate the affects of the interference. As a general rule, the distortion that is generated in the desired channel, when the IF filter bandwidth is momentarily narrowed, is preferable to the interruption that occurs when the adjacent channel is allowed to interfere with the desired channel.
Systems that have employed dynamic IF filter bandwidth control have typically utilized one of two designs. A first design has reduced the IF filter bandwidth in the presence of a strong adjacent channel. A second design has reduced the IF filter bandwidth at low RF and/or low modulation levels. In the second design case, the IF filter bandwidth reduction is typically proportional to the modulation and reception conditions. In both designs, a bandwidth control monitor voltage has normally provided a direct current (DC) representation of the status of the IF filter bandwidth. In systems implementing these designs, a lower bandwidth control monitor voltage usually indicates that the IF filter is operating at maximum bandwidth, whereas a higher bandwidth control monitor voltage typically indicates a narrow IF filter bandwidth. These systems have also normally utilized a linear transition between the minimum and maximum bandwidth control monitor voltage.
Some high quality FM reception systems have also implemented antenna diversity. Antenna diversity systems generally reduce the effects of receiving multipath signals, implement at least two antennas and may include circuitry for combining the signals from multiple antennas. Two basic approaches have normally been utilized within FM reception systems that implement antenna diversity. The first approach is known as switched diversity, where the FM reception system chooses the best antenna and discards received signals from the other antenna(s). The second approach is known as phased array, where the phase of the received signals, from multiple antennas, is aligned and the received signals are combined to provide a composite antenna signal. One known phased array antenna diversity system, that utilizes two antennas, is described in U.S. Pat. No. 5,517,686, which discloses an adaptive reception system (ARS) that amplitude modulates one antenna vector with a set frequency and phase signal. In this system, a detector of an FM radio provides an FM composite signal that is utilized by the ARS to align the received signals. The ARS compares the phase of the FM composite signal with a modulated signal and produces an error voltage to align the phase of one of the received signals with the sum of the two received signals.
In FM reception systems that implement both dynamic IF filter bandwidth control and phased array antenna diversity, the different group delays produced by the IF filter, as its bandwidth is dynamically varied, effects the ability of the ARS to function properly. That is, a reduction in the bandwidth proportionally increases the phase delay of the FM composite signal provided to the ARS. As previously mentioned, the FM composite signal is utilized by the ARS to determine the amount of phase shift that needs to be applied to the received signal from one of the antennas so that the signals received from each antenna are aligned. As currently designed, these FM reception systems cannot differentiate between a phase difference in the received signals and a phase shift caused by dynamically varying the bandwidth of the IF filter and, as such, the performance of these systems may widely vary.
Thus, what is needed is a phase compensation circuit that improves the performance of an FM reception system that implements IF filter bandwidth control in combination with a phased array antenna diversity system.